The present invention relates to a limiter amplifier suitable for use in a radio communication unit, more particularly in a portable communication unit, and such as a paging receiver.
Compactness and light weight are strongly required on this kind of portable communication unit, which uses a limiter amplifier in addition to a frequency discriminator, to demodulate an FM carrier wave.
One example of conventional limiter amplifiers includes a plurality of amplifiers of the grounded-emitter type, connected in cascade by means of coupling capacitors. The amplifier has upper and lower limits which are determined by a saturated collector voltage, i.e., a power source voltage. Therefore, the use of a low-voltage power source, such as a single dry battery, may provide a distorted waveform having asymmetric upper and lower limits at the output of the amplifier.
In addition, such a limiter amplifier cannot be fabricated in a monolithic integrated circuit because coupling capacitors are required. Moreover, since conventional limiter amplifiers have no temperature protection, they involve another problem because their saturation characteristics vary with changes in ambient temperature.